1. Field of the Invention
The present invention relates to image forming apparatus, and, more particularly, to a method of determining a near full condition of an output bin in a printer.
2. Description of the Related Art
An image forming apparatus, such as an electrophotographic printer, typically includes at least one input tray and at least one output bin. Print media of a particular media type, such as plain paper, letterhead, card stock, envelope, label, transparency, pre-printed form, bond and/or color paper is transported from a selected input tray, through the image forming apparatus and into a selected output bin. The print media is typically discharged into the output bin at a location near the top of the output bin. For most print jobs, the depth of the output bin is sufficient to ensure that the output stack of print media does not block the discharge opening near the top of the output bin. However, for particularly large print jobs, the output stack may increase in height to a point such that the output level interferes with the discharge of subsequent sheets of the print medium, thereby possibly causing a paper jam in the printer. Moreover, it is more and more common for printers to be connected to a multi-user network, such as a local area network, in which multiple print jobs can be sent to the printer over a relatively short period of time. Unless the printer is continuously monitored, the multiple print jobs which are printed by the printer may also result in the output stack reaching an output level which is too high, thereby also resulting in a paper jam.
It is known to configure an image forming apparatus with two sensors which are positioned adjacent to an output bin. Such sensors are typically in the form of an optical sensor, although mechanical sensors having a lever arm may also be utilized. One of the sensors is actuated when the output level of the output stack in the output bin reaches a point which is below but relatively close to the full level in the output bin. Upon actuation of this sensor, the printer provides an indication to a user that the output level of the output stack in the output bin has reached a near full level. Such an indication may be a visual indication on a display panel or an audible indication such as an alarm. If the output stack is not removed from the output bin and the output level of the output stack increases within the output bin, the second sensor is positioned adjacent to the output bin to be actuated when the output stack reaches a full level within the output bin. The printer may then either provide another indication to the user that the output stack has further increased in height to the full level and/or temporarily halt operation of the printer pending removal of the output stack.
Utilizing two separate sensors as described above is adequate to provide an indication to the user of the different output levels within the printer and to inhibit paper jams associated with an output level at the full level. However, the necessity to use two separate sensors adds to the complexity and cost of the printer. Moreover, the microprocessor within the printer may need separate inputs which are respectively connected with the two sensors so that the signals may be received therefrom. The possible need for an increased number of inputs on the microprocessor also adds to the complexity and cost of the printer.
It is also known to utilize a single sensor within a printer which is positioned adjacent to an output bin and senses a near full level of the output stack within the output bin. A user may be provided with an indication that the output level of the output stack has reached a near full level upon actuation of the single sensor. Rather than utilizing a second sensor to sense the full level of the output stack, the microprocessor is configured such that a predefined number of print media sheets may be transported to the output bin after the near full level of the output stack has been sensed. The predetermined number is typically based upon an average thickness of a media type which is normally printed by the printer. For example, most print jobs require the use of plain paper with a 20 pound basis weight. Plain paper has an average thickness of about 0.004 inch. If the output bin has a maximum full level of about 500 sheets, the sensor may be positioned at an output level of the output stack corresponding to about 450 print media sheets and the predetermined number may be set to 50 such that the full level occurs after 50 print media sheets have been transported into the output bin after actuation of the near full level sensor.
A printer using a single sensor to detect the near full level of the output stack as described above works properly if the media type of the print media corresponds to plain paper. However, other media types may stack differently within the output stack in the output bin because of physical characteristics associated therewith. For example, envelopes have a thickness which is greater than the thickness of plain paper because of being folded. It is therefore not possible to transport as many envelopes into the output bin after the near full level has been sensed when compared to plain paper. On the other hand, other types of plain paper may have a basis weight and thickness which is less than 20 pound paper. It is therefore possible with thinner paper to transport more than the predetermined number of sheets into the output bin. If the predetermined number is based upon 20 pound paper, the output bin may not actually be fully utilized at the point when the full level is inferred. Moreover, certain media types have a tendency to curl after being transported through the printer and into the output bin. The print media may curl around an axis of symmetry which extends around a longitudinal axis of the print media sheet, or crosswise to the longitudinal axis of the print media sheet. The tendency of the print media to curl results in an "effective" height of the print media in the output bin which is greater than the actual thickness of the single print media sheet. A print job utilizing a print media which has a tendency to curl results in an output stack having an effective output level which is greater than the theoretical output level of the accumulated thicknesses of the sheets. Transporting the predetermined number of print media sheets having a tendency to curl therefore may result in the effective output level of the output stack being greater than the full level, thereby possibly causing paper jams in the printer.
Fig. 1 illustrates an output stack of print media exhibiting curl with an axis of symmetry about the longitudinal axis of the print media sheets. The effective output level of the output stack is a function of the curl factor, represented by the quotient of the effective thickness of the accumulated sheets (D.sub.CURL) divided by the theoretical thickness of the accumulated print media sheets (D.sub.VIRGIN).
What is needed in the art is an image forming apparatus which does not require multiple sensors for determination of near full and full output levels, and which more accurately predicts when the full level has been reached within an output bin.